The present invention relates to a magnetic recording medium, in particular, one having a back coat with improved surface properties.
Magnetic recording media such as magnetic tape, magnetic sheet and magnetic disk are used extensively in audio, video and computer applications. Video tapes are often encased in cassettes. When recording or reproducing image with video tapes, a cassette is loaded on a video deck and the tape is scanned with the magnetic head as it runs in abrasive contact with guide poles and rollers. For successful image recording and reproduction, the tape must run normally and its winding must be uniform enough to permit cyclic use. Since the tape runs at high speed and because it is usually made of materials having high electrical resistance, the surface properties of the tape which makes abrasive contact with guide poles and rollers are required to be appropriate for tape running and to have antistatic effect.
On the other hand, in order to increase sensitivity and especially to improve the output in the high-frequency range, the magnetic layer of magnetic tape is finished to have a smooth surface so that the tape will maintain a constant state of abrasion with the magnetic head. However, if the surface of the magnetic layer is very smooth, its dynamic frictional resistance increases to cause occasional problems during tape running.
Further, when magnetic tape is caused to run on a video deck, not only its obverse surface but also its reverse surface undergoes abrasion by guide poles and rollers. Therefore, if the running property and the durability of the back surface of the tape are not good, an irregular and sudden tension will be exerted upon the running tape to cause excessive abrasion of the magnetic layer by the head. As a result, not only is the magnetic layer damaged but also the shedding of magnetic particles will occur. In addition, the tension at which the tape is wound varies constantly, causing unevenness in winding pressure and the form in which tape is wound up. As a result, the edges of turns of wound tape becomes nonuniform and this results in uneven running in subsequent use of the tape. All these phenomena contribute to deterioration of image and the electromagnetic characteristics of the tape as typified by skew, jitter and low S/N ratio.
With a view to overcoming these problems, it has been proposed that a back coat be provided on the reverse side of magnetic tape. An example of such proposal is the inclusion of a inorganic powder in a resin layer. This approach is intended to roughen the surface of the back coat so that the area of contact with guide poles and rollers is sufficiently reduced to realize a lower coefficient of friction. For instance, JP-A-57-53825 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-58-2415 show examples of using inorganic powders. However, in these references, the disclosure on Mohs scale of hardness is not made sufficiently, therefore, if such inorganic powder was used in a wrong way, performance of the resulting magnetic tape deteriorates.
The use of carbon black in place of inorganic powders has also been proposed as in JP-B-52-17401 (the term "JP-B" as used herein means an "examined Japanese patent publication"). The technique disclosed in this publication aims at preventing static buildup by the electroconductivity of carbon black, as well as the light-shielding and surface-roughening effects of its particles. However, the carbon black particles used have an average size of 10-20 .mu.m and hence have very low dispersibility in paints or coating solutions. When a back coat is formed by applying the resulting dispersion, the carbon black particles will agglomerate and produce coarse asperities on the surface. Further, such agglomerated particles adhere so weakly to the binder that they will easily shed off the surface. In addition, when the tape is wound and if adjacent turns contact each other, large agglomerated particles in the back coat of one layer will produce asperities on the magnetic coating of the adjacent layer.
Thus, carbon black particles having a small size are poor in dispersibility and it has been difficult to obtain a back coat that has such a mean roughness as to avoid the formation of asperities on the magnetic coating of an adjacent layer. In other words, the use of such small particles has had a tendency to produce an excessively coarse surface.
In an attempt to reduce the frictional resistance of back coats, it has been proposed that a carbon black powder having a fairly small average primary particle size of 10-60 .mu.m be used in combination with a carbon black powder having an average primary particle size of more than 100 .mu.m (see, for example, JP-A-60-45938, JP-A-60-25023, JP-A-59-185027 and JP-A-59-223937. However, satisfactory improvements could not be achieved in running property and durability by merely combining carbon black powders of different particle sizes.
It has also been proposed that synergism be realized by using an inorganic powder in combination with carbon black (see JP-A-59-210534, JP-A-60-25022 and JP-60-25023) but, in this approach, particle size of the carbon black and the Mohs scale of hardness of the inorganic powder must be selected so as to adapt one to the other.
The binder to be used in back coats is no less important than that used in the magnetic layer and involves many factors to be taken into account such as strength, adhesion to fillers, its ability to disperse fillers, surface properties and ease of handling, in particular, the applicability of coating solutions that contain it. In current practice, the binders used in the magnetic layer is simply applied to the back coat and in the absence of thorough reviews on binders that are particularly suitable for use in the back coat, there is a high likelihood that the back coat which is deliberately provided to improve the characteristics of magnetic recording media will simply add to the chance of increased occurrence of defects in the media. To take nitrocellulose as an exemplary binder, the dispersibility of fillers is reduced if the molecular weight of nitrocellulose is high and agllomeration will occur. If, on the other hand, the molecular weight of nitrocellulose is low, the dipersibility of the filler is satisfactory but then it is not durable and will easily shed off.